Patent Literature 1 below discloses a technique for driving a synchronous motor having a multi-group winding structure including a plurality of three-phase windings. In the disclosed technique, a synchronous motor has two groups of windings, one group driven by a master inverter and another group driven by a slave inverter, and both invertors calculate an estimated phase angle from a voltage command related to the rotating coordinates and a current detection signal as well as from the circuit constant and the estimated speed of the motor; an average calculation unit obtains the average of the estimated phase angles; a speed estimation unit calculates an estimated speed from the average; and a position integration unit calculates an estimated reference phase by performing temporal integration on the estimated speed.